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Chinese VoIP Kit Contains Backdoor, Warn Researchers

Researchers at Trustwave are warning of a hidden backdoor in VoIP devices produced by Chinese manufacturer DBL Technology which could allow access by the manufacturer or malicious third parties.

The issue is with the authentication process, allowing a remote attacker to gain a shell with root privileges on an affected device, Trustwave researcher Neil Kettle explained in a blog post.

“The Telnet interface of the GoIP is documented as providing information for users of the device through the use of logins ‘ctlcmd’ and ‘limitsh’. Both of these logins provide limited information about the device, and are accessed using the user-configured administrator password. However, an additional undocumented user, namely ‘dbladm’ is present which provides root level shell access on the device. Instead of a traditional password, this account is protected by a proprietary challenge-response authentication scheme,” he explained.

“Investigation has shown this scheme to be fundamentally flawed in that it is not necessary for a remote user to possess knowledge of any secret besides the challenge itself and knowledge of the protocol/computation.”

This is apparently in contrast to more secure challenge-response schemes such as password-based log-ins where the user is asked for a password, which is then obscured to guard against “network interception and replay attacks.”

The issue was first spotted by Trustwave in an 8 port VoIP GSM Gateway from the company. However, it’s since been discovered present in GoIP 1, 4, 8, 16 and 32 and could affect many more DBL Technology devices and OEM kit.

More worryingly, when contacted last October, the firm did not fix the issue.

“Verification of the patched version reveals that the challenge response mechanism is still present in the latest version albeit a little more complex. It seems DBL Technology engineers did not understand that the issue is the presence of a flawed challenge response mechanism and not the difficulty of reverse engineering it,” explained Kettle.

“The main differences between the latest challenge response mechanism and the older variant is the level of complexity it employs: a simplistic MD5 with a linear equation changed to several 'round' functions mixed with a modified version of the MD5 hash algorithm.”

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